For Your Information ...
Total Harmonic Distortion of
Electronic Fluorescent Dimming Ballasts
APPLICATION
NOTE
#113
Overview
Line current is delivered at 60 Hz (the fundamental frequency of line voltage) and some current is also
delivered at other frequencies that are harmonics (multiples) of 60 Hz. No power is delivered to the
electronic device at these other frequencies.
Total Harmonic Distortion (THD) is a measure of the magnitude of all the harmonics of the input
current, expressed as a percentage of the fundamental frequency current.
Discussion
Line current is supplied to electrical equipment at a fundamental frequency of 60Hz. Harmonics to
the fundamental current are integer multiples of the fundamental frequency - the 2nd harmonic is at
120Hz, and the 3rd harmonic is at 180Hz, etc. Depending on the electrical design of the equipment,
it can distort the input sine wave of the fundamental frequency and introduce harmonics on the power
line.
Why high THD is detrimental
Significant harmonics are put back on the line if the load type is not linear and draws current from
the line in short pulses. Harmonic currents do not provide any power to the load, and flow in the
neutral wire in a 3-phase system. Harmonics up to 33% are allowed by ANSI.
When line harmonics are present, the total current is the summation of the fundamental frequency
current and the harmonic current. If the THD of an electrical device is 20%, it introduces 20% of the
magnitude of its input current back on the line as harmonics. If this device constitutes 10% of the
total electric load in a building, then it contributes only 2% to the total building THD. Codes and
regulations require that total building THD not exceed 33%.
THD may be a concern for 2-wire line-voltage dimming ballasts as the power wires carry the phasecontrol dimming signal to the ballast. The current drawn by the 2-wire dimming ballast mimics the
phase-control dimming signal and will result in higher THD when dimmed.
THD regulations
In the early 1980s when there did not exist a standard for THD of electronics, some dimming ballasts
would produce up to 100% THD when dimmed down to low light levels. There have been many
design improvements since, and ANSI C82.11-1993 now specifies maximum 33% THD for electronic
ballasts.
Lutron dimming ballasts have a THD rating of 20% or less, with our premium Hi-lume ballasts
rated at 10%.
Need additional assistance? Call the Lutron Technical Support Center 1-800-523-9466.
Please provide exact model number when calling.
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Summary
THD of equipment is an important consideration for electrical safety but in most buildings, lighting
amounts to less than 30% of total building load, which translates to less than 10% of total building
THD. Limiting the THD of all lighting fixtures to below the ANSI regulated 33% is therefore sufficient
to ensure a robust electrical power supply in any building. All Lutron dimming ballasts are rated for
20% THD or less.
Worldwide Technical and Sales Assistance
If you need assistance call the toll-free Lutron Technical
Support Center. 24 hours/day + 7 days/week
Please provide exact model number when calling.
(U.S.A. and Canada) (800) 523-9466
Other countries call (610) 282-3800
Visit us on the web at www.lutron.com
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7200 Suter Road
Coopersburg, PA 18036-1299 U.S.A.
Phone: (610) 282-3800
Fax:
(610) 282-3090
Lutron is a registered trademark of Lutron Electronics Co., Inc.
© 2004 Lutron Electronics, Co., Inc.
Need additional assistance? Call the Lutron Technical Support Center 1-800-523-9466.
Please provide exact model number when calling.
Lutron Electronics Co., Inc.
7200 Suter Road
Coopersburg, PA 18036-1299 U.S.A.
Made and printed in U.S.A. 3/04 P/N 048-075 Rev. A